Android Low Memory Killer

转自：http://www.cnblogs.com/angeldevil/archive/2013/05/21/3090872.html
Low Memory Killer的原理

　　在Android中，即使当用户退出应用程序之后，应用程序的进程也还是存在于系统中，这样是为了方便程序的再次启动，但是这样的话，随着打开的程序数量的增加，系统的内存会变得不足，就需要杀掉一部分进程以释放内存空间。至于是否需要杀死一些进程和哪些进程需要被杀死，是通过Low Memory Killer机制来进行判定的。

　　Android的Low Memory Killer基于Linux的OOM机制，在Linux中，内存是以页面为单位分配的，当申请页面分配时如果内存不足会通过以下流程选择bad进程来杀掉从而释放内存：

alloc_pages -> out_of_memory() -> select_bad_process() -> badness()

　　在Low Memory Killer中通过进程的oom_adj与占用内存的大小决定要杀死的进程，oom_adj越小越不容易被杀死。

　　Low Memory Killer Driver在用户空间指定了一组内存临界值及与之一一对应的一组oom_adj值，当系统剩余内存位于内存临界值中的一个范围内时，如果一个进程的oom_adj值大于或等于这个临界值对应的oom_adj值就会被杀掉。

　　可以通过修改/sys/module/lowmemorykiller/parameters/minfree与/sys/module/lowmemorykiller/parameters/adj来改变内存临界值及与之对应的oom_adj值。minfree中数值的单位是内存中的页面数量，一般情况下一个页面是4KB。
　　比如如果向/sys/module/lowmemorykiller/parameters/adj写入0,8，向/sys/module/lowmemorykiller/parameters/minfree中写入1024,4096，假设一个页面大小为4KB，这样当系统空闲内存位于1024*4~4096*4KB之间时oom_adj大于等于8的进程就会被杀掉。

　　在lowmemorykiller.c中定义了阈值表的默认值，可以通过init.rc自定义：
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static int lowmem_adj[6] = {        0,        1,        6,        12,};static int lowmem_adj_size = 4;static size_t lowmem_minfree[6] = {        3 * 512,        /* 6MB */        2 * 1024,       /* 8MB */        4 * 1024,       /* 16MB */        16 * 1024,      /* 64MB */};static int lowmem_minfree_size = 4;　复制代码　　在init.rc中定义了init进程的oom_adj为-16，不可能会被杀死(init的PID是1)：on early-init    # Set init and its forked children's oom_adj.    write /proc/1/oom_adj -16　　在Linux中有一个kswapd的内核线程，当linux回收内存分页的时候，kswapd线程将会遍历一张shrinker链表，并执行回调，定义如下：复制代码/* * A callback you can register to apply pressure to ageable caches. * * 'shrink' is passed a count 'nr_to_scan' and a 'gfpmask'.  It should * look through the least-recently-used 'nr_to_scan' entries and * attempt to free them up.  It should return the number of objects * which remain in the cache.  If it returns -1, it means it cannot do * any scanning at this time (eg. there is a risk of deadlock). * * The 'gfpmask' refers to the allocation we are currently trying to * fulfil. * * Note that 'shrink' will be passed nr_to_scan == 0 when the VM is * querying the cache size, so a fastpath for that case is appropriate.*/struct shrinker {    int (*shrink)(int nr_to_scan, gfp_t gfp_mask);    int seeks;      /* seeks to recreate an obj */    /* These are for internal use */    struct list_head list;    long nr;        /* objs pending delete */};#define DEFAULT_SEEKS 2 /* A good number if you don't know better. */extern void register_shrinker(struct shrinker *);extern void unregister_shrinker(struct shrinker *);

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　　通过register_shrinker与unregister_shrinker向shrinker链表中添加或移除回调。当注册Shrinker后就可以在回收内存分页时按自己定义的规则释放内存。

　　Android Low Memory Killer的代码在drivers/staging/android/lowmemorykiller.c中，通过以下代码在模块初始化时注册Shrinker：
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static int lowmem_shrink(int nr_to_scan, gfp_t gfp_mask);static struct shrinker lowmem_shrinker = {        .shrink = lowmem_shrink,        .seeks = DEFAULT_SEEKS * 16};static int __init lowmem_init(void){        register_shrinker(&lowmem_shrinker);        return 0;}static void __exit lowmem_exit(void){        unregister_shrinker(&lowmem_shrinker);}module_init(lowmem_init);module_exit(lowmem_exit);

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　　这样就可以在回收内存分页时调用lowmem_shrink函数。
Low Memory Killer的实现

　　lowmem_shrink的定义如下：
View Code

　　分开来看这段代码，首先取得内存阈值表的大小，取阈值表数组大小与lowmem_adj_size，lowmem_minfree_size的较小值，然后通过globa_page_state获得当前剩余内存的大小，然后跟内存阈值表中的阈值相比较获得min_adj与selected_oom_adj：
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int array_size = ARRAY_SIZE(lowmem_adj);int other_free = global_page_state(NR_FREE_PAGES);int other_file = global_page_state(NR_FILE_PAGES);if (lowmem_adj_size < array_size)        array_size = lowmem_adj_size;if (lowmem_minfree_size < array_size)        array_size = lowmem_minfree_size;for (i = 0; i < array_size; i++) {    if (other_free < lowmem_minfree[i] && other_file < lowmem_minfree[i]) {         min_adj = lowmem_adj[i];         break;    }}selected_oom_adj = min_adj;

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　　遍历所有进程找到oom_adj>min_adj并且占用内存大的进程：
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read_lock(&tasklist_lock);for_each_process(p) {    struct mm_struct *mm;    int oom_adj;    task_lock(p);    mm = p->mm;    if (!mm) {        task_unlock(p);        continue;    }    oom_adj = mm->oom_adj;    //获取task_struct->struct_mm->oom_adj，如果小于警戒值min_adj不做处理    if (oom_adj < min_adj) {        task_unlock(p);        continue;    }    //如果走到这里说明oom_adj>=min_adj，即超过警戒值    //获取内存占用大小，若<=0，不做处理    tasksize = get_mm_rss(mm);    task_unlock(p);    if (tasksize <= 0)        continue;    //如果之前已经先择了一个进程，比较当前进程与之前选择的进程的oom_adj与内存占用大小，如果oom_adj比之前选择的小或相等而内存占用比之前选择的进程小，不做处理。    if (selected) {        if (oom_adj < selected_oom_adj)            continue;        if (oom_adj == selected_oom_adj &&            tasksize <= selected_tasksize)            continue;    }    //走到这里表示当前进程比之前选择的进程oom_adj大或相等但占用内存大，选择当前进程    selected = p;    selected_tasksize = tasksize;    selected_oom_adj = oom_adj;    lowmem_print(2, "select %d (%s), adj %d, size %d, to kill\n",                 p->pid, p->comm, oom_adj, tasksize);}

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如果选择出了符合条件的进程，发送SIGNAL信号Kill掉：
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if (selected) {    lowmem_print(1, "send sigkill to %d (%s), adj %d, size %d\n",                 selected->pid, selected->comm,                 selected_oom_adj, selected_tasksize);    force_sig(SIGKILL, selected);    rem -= selected_tasksize;}

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oom_adj与上层Process Importance的关系
我们知道，在上层进程按重要性可以分为：Foreground process，Visible process，Service process，Background process与Empty process，那么这些重要性怎么与Low Memory Killer中的oom_adj对应起来的呢？
在ActivityManager.RunningAppProcessInfo中我们可以看到如下关于importance的定义：
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/**
* Constant for {@link #importance}: this is a persistent process.
* Only used when reporting to process observers.
* @hide
*/
public static final int IMPORTANCE_PERSISTENT = 50;

/**
* Constant for {@link #importance}: this process is running the
* foreground UI.
*/
public static final int IMPORTANCE_FOREGROUND = 100;

/**
* Constant for {@link #importance}: this process is running something
* that is actively visible to the user, though not in the immediate
* foreground.
*/
public static final int IMPORTANCE_VISIBLE = 200;

/**
* Constant for {@link #importance}: this process is running something
* that is considered to be actively perceptible to the user. An
* example would be an application performing background music playback.
*/
public static final int IMPORTANCE_PERCEPTIBLE = 130;

/**
* Constant for {@link #importance}: this process is running an
* application that can not save its state, and thus can’t be killed
* while in the background.
* @hide
*/
public static final int IMPORTANCE_CANT_SAVE_STATE = 170;

/**
* Constant for {@link #importance}: this process is contains services
* that should remain running.
*/
public static final int IMPORTANCE_SERVICE = 300;

/**
* Constant for {@link #importance}: this process process contains
* background code that is expendable.
*/
public static final int IMPORTANCE_BACKGROUND = 400;

/**
* Constant for {@link #importance}: this process is empty of any
* actively running code.
*/
public static final int IMPORTANCE_EMPTY = 500;

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　　这些常量表示了Process的Importance等级，而在ProcessList中我们会发现关于adj的一些定义：
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// This is a process only hosting activities that are not visible,
// so it can be killed without any disruption.
static final int HIDDEN_APP_MAX_ADJ = 15;
static int HIDDEN_APP_MIN_ADJ = 9;

// The B list of SERVICE_ADJ – these are the old and decrepit
// services that aren’t as shiny and interesting as the ones in the A list.
static final int SERVICE_B_ADJ = 8;

// This is the process of the previous application that the user was in.
// This process is kept above other things, because it is very common to
// switch back to the previous app. This is important both for recent
// task switch (toggling between the two top recent apps) as well as normal
// UI flow such as clicking on a URI in the e-mail app to view in the browser,
// and then pressing back to return to e-mail.
static final int PREVIOUS_APP_ADJ = 7;

// This is a process holding the home application – we want to try
// avoiding killing it, even if it would normally be in the background,

// because the user interacts with it so much.
static final int HOME_APP_ADJ = 6;

// This is a process holding an application service – killing it will not
// have much of an impact as far as the user is concerned.
static final int SERVICE_ADJ = 5;

// This is a process currently hosting a backup operation. Killing it
// is not entirely fatal but is generally a bad idea.
static final int BACKUP_APP_ADJ = 4;

// This is a process with a heavy-weight application. It is in the
// background, but we want to try to avoid killing it. Value set in
// system/rootdir/init.rc on startup.
static final int HEAVY_WEIGHT_APP_ADJ = 3;

// This is a process only hosting components that are perceptible to the
// user, and we really want to avoid killing them, but they are not
// immediately visible. An example is background music playback.
static final int PERCEPTIBLE_APP_ADJ = 2;

// This is a process only hosting activities that are visible to the
// user, so we’d prefer they don’t disappear.
static final int VISIBLE_APP_ADJ = 1;

// This is the process running the current foreground app. We’d really
// rather not kill it!
static final int FOREGROUND_APP_ADJ = 0;

// This is a system persistent process, such as telephony. Definitely
// don’t want to kill it, but doing so is not completely fatal.
static final int PERSISTENT_PROC_ADJ = -12;

// The system process runs at the default adjustment.
static final int SYSTEM_ADJ = -16;

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　　我们可以看到：

static final int PREVIOUS_APP_ADJ = 7;
static final int HOME_APP_ADJ = 6;

　　并不是所有的Background process的等级都是相同的。

　　关于ADJ与Importance的值都找到了，那么它们是怎么对应起来的呢？Activity实际是由ActivityManagerService来管理的，在ActivityManagerService中我们可以找到以下函数：
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static int oomAdjToImportance(int adj, ActivityManager.RunningAppProcessInfo currApp) {
if (adj >= ProcessList.HIDDEN_APP_MIN_ADJ) {
if (currApp != null) {
currApp.lru = adj - ProcessList.HIDDEN_APP_MIN_ADJ + 1;
}
return ActivityManager.RunningAppProcessInfo.IMPORTANCE_BACKGROUND;
} else if (adj >= ProcessList.SERVICE_B_ADJ) {
return ActivityManager.RunningAppProcessInfo.IMPORTANCE_SERVICE;
} else if (adj >= ProcessList.HOME_APP_ADJ) {
if (currApp != null) {
currApp.lru = 0;
}
return ActivityManager.RunningAppProcessInfo.IMPORTANCE_BACKGROUND;
} else if (adj >= ProcessList.SERVICE_ADJ) {
return ActivityManager.RunningAppProcessInfo.IMPORTANCE_SERVICE;
} else if (adj >= ProcessList.HEAVY_WEIGHT_APP_ADJ) {
return ActivityManager.RunningAppProcessInfo.IMPORTANCE_CANT_SAVE_STATE;
} else if (adj >= ProcessList.PERCEPTIBLE_APP_ADJ) {
return ActivityManager.RunningAppProcessInfo.IMPORTANCE_PERCEPTIBLE;
} else if (adj >= ProcessList.VISIBLE_APP_ADJ) {
return ActivityManager.RunningAppProcessInfo.IMPORTANCE_VISIBLE;
} else {
return ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND;
}
}

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　　在这个函数中实现了根据adj设置importance的功能。

　　我们还可以看到SERVICE还分为SERVICE_B_ADJ与SERVICE_ADJ，等级是不一样的，并不是所有Service的优先级都比Background process的优先级高。当调用Service的startForeground后，Service的importance就变为了IMPORTANCE_PERCEPTIBLE（在记忆中曾经将Service设置为foreground并打印出其importance的值与IMPORTANCE_PERCEPTIBLE相等），对应的adj是PERCEPTIBLE_APP_ADJ，即2，已经很难被系统杀死了。
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// This is a system persistent process, such as telephony. Definitely
// don’t want to kill it, but doing so is not completely fatal.
static final int PERSISTENT_PROC_ADJ = -12;

// The system process runs at the default adjustment.
static final int SYSTEM_ADJ = -16;

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　　像电话等进程的adj为-12已基本不可能被杀死了，而在前面已经看到了，init.rc中将init进程的oom_adj设置为了-16，已经是永生进程了。